78 THE SCIENCE OF LIFE. 



of the essential differences between living beings, since 

 the plan of structure varies for attaining similar purpose. 

 7. Cuvier proposed four primary divisions of the ani- 

 mal kingdom, because, he said, they are constructed on 

 four different plans. These plans may be briefly stated 

 as follows : " In the Vertebrates there is a vertebral col- 

 umn terminating in a prominent head ; this column has 

 an arch above and an arch below, forming a double in- 

 ternal cavity. The parts are symmetrically arranged on 

 either side of the longitudinal axis of the body. In the 

 MolluskS) also, the parts are arranged according to a bi- 

 lateral symmetry on either side of the body, but the 

 body has but one cavity, and is a soft, concentrated 

 mass, without a distinct individualization of parts. In 

 the Articulates there is but one cavity, and the parts are 

 here again arranged on either side of the longitudinal 

 axis, but in these animals the whole body is divided 

 from end to end in transverse rings or joints movable 

 upon each other. In the Radiates we lose sight of the 

 bilateral symmetry so prevalent in the other three, ex- 

 cept as a very subordinate element of structure; the 

 plan of this lowest type is an organic sphere, in which 

 all parts bear definite relations to a vertical axis." * 

 Leuckart proposed to subdivide the Radiates into two 

 groups ; the Coelenterata, including Polyps and Acalephs, 

 or Jelly-fishes and Echinoderms, including Star-fishes, 

 Sea-Urchins, and Holothurians, but Agassiz shows that 

 the differences between them are not differences in the 

 plan, but merely a difference in the execution of the 

 plan, since both are equally radiate in structure. 

 * Agassiz, " Methods of Study." 



